The present invention relates to satellite communication systems and, particularly to geostationary satellite systems that utilize non-geostationary satellite frequency spectrum in a non-interfering manner.
Satellites are either in geostationary orbit (GSO), i.e., stationary relative to the earth, or in non-geostationary orbit (NGSO), traveling around the earth. In general, the frequency bands allocated to GSO satellite communication systems do not overlap with the frequency bands allocated to NGSO satellite communication systems.
Radio frequency (RF) spectrum is a limited finite resource. Only certain frequency bands are allocated to GSO satellite communication systems. Some other frequency bands are allocated to NGSO satellite communication systems. Channel capacity of any communication system is limited by the number of frequency bands and the associated available bandwidth. There is a need for a GSO satellite system to utilize frequency bands allocated to other wireless communication systems in order to obtain higher channel capacity without causing any interference.
A GSO satellite is in orbit about 35,800 km above the equator, and its revolution around the earth is synchronized with the earth's rotation. Therefore, the GSO satellite appears stationary, i.e., fixed in the sky to an observer on the earth's surface. Unlike GSO satellites, NGSO satellites typically travel at low and medium altitudes and have variable orbits that are below the GSO orbit. A GSO Earth terminal with a narrow antenna beam width will have its antenna beam pointed at a GSO satellite. Thus, an NGSO satellite will only be visible to the GSO Earth terminal when it is “in-line” with respect to the GSO Earth terminal and the GSO satellite. Similarly, an NGSO ground station with a narrow antenna beam width will have its antenna beam pointed at the NGSO satellite. Since NGSO satellites are non-stationary, the NGSO ground station's antenna may be steerable in order to follow the NGSO satellite. The GSO satellite will only be visible to the NGSO ground station when the GSO and the NGSO satellites are “in-line” or approximately “in-line.”
Prior art GSO satellite communication systems utilize only frequency bands that are allocated to GSO satellite systems. The allocated GSO frequency bands differ from those allocated to NGSO satellite systems in order to avoid interference. In certain allocated frequency spectra, the NGSO frequency bands may be allocated in proximity to the GSO frequency bands.
The GSO satellite may employ a multi-beam antenna that illuminates certain areas of the Earth's surface. Therefore, the beam cone of the satellite antenna is relatively wide in order to provide a large coverage area. By contrast, the cone shape of an uplink beam from an Earth terminal antenna to the GSO satellite is in general a very narrow beam. The beam (also referred to hereafter as a channel) relayed from a satellite to a controlling ground station a.k.a. an Earth terminal is called a downlink beam (or downlink channel) and the beam from the Earth terminal to the satellite is called an uplink beam (or uplink channel). Different frequency bands are allocated for the uplink channel and for the downlink channel to prevent co-channel interference. As the orbital trajectory of an NGSO satellite may cross the uplink or downlink channel of a GSO satellite communication system, frequency bands differing from the GSO frequency bands have in the past been allocated to the NGSO satellite. However, depending on the characteristics of the NGSO satellite constellation (e.g., low Earth orbit, medium Earth orbit), the time period during which the NGSO satellite is between the GSO satellite and the ground station, that is, whenever it is substantially “in-line” with the GSO satellite and the ground station of interest, is relative short, so that allocated NGSO frequency bands are temporally underutilized when the NGSO satellite is not in-line. Moreover, currently deployed NGSO satellites may not use certain allocated NGSO frequency bands for operation, so that those NGSO frequency bands are always underutilized.